In B cell lymphoid malignancies, malignant cells proliferate monoclonally with the same immunoglobulin gene rearrangement.This rearrangement can be detected by the use of the polymerase chain reaction(PCR).Recently, it has been reported that proliferation of clonal cells could be demonstrated by PCR analyzing DNA extracted from patients' plasma with B cell malignancies.The question, however, still remains whether amplified products were derived from tumor cells or not.In this study, using DNA extracted from both malignant cells and plasma of the same patients with B cell leukemia, I attempted to amplify immunoglobulin heavy chain(IgH) complementarity determining region 3(CDR3) by semi-nested PCR and to analyze the sequences respectively.The results showed that sequences of DNA from plasma were identical with that of DNA from tumor cells in all cases.In addition, some of them showed homology to CDR3 that was previously reported.These findings indicate that plasma DNA is released from tumor cells.In the next stage, it is necessary to raise the sensitivity, and if it works, IgH-PCR using plasma DNA might be useful tool for diagnosing patients and evaluating the efficacy of treatment, especially in patients with critical condition under which any biopsies were difficult.